1. Field of the Invention
The present invention relates to a lens support structure, a lens barrel, and a camera, and more particularly relates to a lens support structure, a lens barrel, and a camera in which a plurality of lenses of different materials are supported.
2. Description of the Related Art
Digital cameras, which have become very popular in recent years, make use of a CCD (charge coupled device) or CMOS (complementary metal-oxide semiconductor) sensor or another such imaging element to convert an optical image into an electrical signal, and to digitize and record the electrical signal.
In the field of digital cameras, the main body needs to be made smaller to make the product easier to carry. More specifically, reducing the size of the lens barrel is believed to contribute greatly to reducing the size of the main body. One possible way to reduce the size of a lens barrel in the direction of its optical axis is to shorten the distance between the lenses.
There is a limit to the dimensional precision of a lens frame. Consequently, when a lens is positioned using the lens frame as a reference, there is a decrease in lens positioning precision.
In view of this, there has been proposed a lens support structure in which a plurality of lenses are brought into direct contact in the direction along the optical axis. A lens face is machined to a higher precision than a lens frame, and therefore, bringing lenses into direct contact allows the lens positioning precision to be increased and the distance between lenses to be shortened.
Meanwhile, optical systems such as these do not always use glass lenses, and plastic lenses are also employed with an eye toward manufacturing costs.
However, plastics and other such resins generally have lower stiffness and a larger coefficient of linear expansion than glass. Therefore, when a plastic lens is fitted into a lens frame with no gap, if the temperature should rise so that the outside diameter of the plastic lens increases, for example, the outer peripheral part of the plastic lens will be pressed by the lens frame, and as a result the plastic lens deforms and its optical performance degrades.
In view of this, a lens support structure in which a gap is ensured between the lens frame and the outer peripheral part of a plastic lens has been proposed in Japanese Laid-Open Patent Applications H8-220408 and H9-281374 in order to prevent degradation of optical performance.
As a result, a change in the outside diameter of the plastic lens caused by temperature change will be absorbed by the gap, so even if the temperature does change, optical performance will not degrade.
Nevertheless, with a conventional lens frame, the inside diameter of the portion that supports a glass lens is the same as the inside diameter of the portion where a plastic lens is disposed inside the inner periphery thereof. Therefore, the only time this structure can be used is when the outside diameter of the glass lens is greater than the outside diameter of the plastic lens. That is, when a plurality of lenses made of different materials are supported with a lens support structure such as this, there is a decrease in design freedom.
Also, the dimensions of a lens support portion are generally corrected to keep the gap in the radial direction between the lens support portion and the lens to a specific amount, or to dispose the lens coaxially.
With the above-mentioned lens support structure, however, since the inside diameters of the portions where the lenses are disposed are all the same, it is difficult to correct the lens support portion for each lens. As a result, there is the risk that dimensional precision of the lens support portion will decrease, and that optical performance will degrade.
Thus, when a plurality of lenses of different materials are supported, with a conventional lens support structure it is difficult to prevent degradation in the optical performance while increasing design freedom.